Tunable CO2 electroreduction to ethanol and ethylene with controllable interfacial wettability

The mechanism of how interfacial wettability impacts the CO2 electroreduction pathways to ethylene and ethanol remains unclear. This paper describes the design and realization of controllable equilibrium of kinetic-controlled *CO and *H via modifying alkanethiols with different alkyl chain lengths to reveal its contribution to ethylene and ethanol pathways. Characterization and simulation reveal that the mass transport of CO2 and H2O is related with interfacial wettability, which may result in the variation of kinetic-controlled *CO and *H ratio, which affects ethylene and ethanol pathways. Through modulating the hydrophilic interface to superhydrophobic interface, the reaction limitation shifts from insufficient supply of kinetic-controlled *CO to that of *H. The ethanol to ethylene ratio can be continuously tailored in a wide range from 0.9 to 1.92, with remarkable Faradaic efficiencies toward ethanol and multi-carbon (C2+) products up to 53.7% and 86.1%, respectively. A C2+ Faradaic efficiency of 80.3% can be achieved with a high C2+ partial current density of 321 mA cm−2, which is among the highest selectivity at such current densities.

• The potential impact on the reaction pathway is important. By saying potential, we mean the potential on the surface of the catalyst. However, ignoring the iR potential loss cannot give us a realistic picture of what potential exists at the cathode. Therefore, the whole discussion in this manuscript can go under question that perhaps all observations are due to the potential shift, and that's why we see different product distribution. EIS test must be performed to measure the impedance of the system and also to elaborate more on the charge transfer resistance for each catalyst.
• In the introduction, one of the main challenges in the field is discussed as ethylene/ethanol selectivity control, however, throughout the manuscript, ethylene is replaced by C2+ products invariantly which makes the story difficult to follow and to draw a clear conclusion.
• To further support the hypothesis that the surface wettability is the only parameter affecting the selectivity (at least in this study), the authors need to repeat the experiment with a different control sample with a given hydrophobicity. For example, applying PTFE on the surface, measuring the contact angel, running the CO2RR, and monitoring the ethanol/ethylene ratio.
A few minor comments: • Line 120: nano-features are not visible in Supplementary Fig. 2. The scale bar in SEM images is in micro scale.
• Fig. 3c: the results for Cu-12C are not shown.
• There are several typos, grammatical mistakes, and difficult sentences to read, even wrong figure numbers (most Fig. 2's must be Fig. 3's). The manuscript needs to be proofread again before resubmission.
• Line 129-130: a table of deconvoluted peaks for XPS results could be provided in SI. Also, a survey can be added.
• Line 129-130: For Cu XPS results, did authors etch the surface of the sample? Cu surface can be easily oxidized (to Cu2+) in the atmosphere, and that's why they observed a little amount of Cu2+ for the Cu sample. This reviewer would suggest etching the surface of the sample first, then running XPS for a better comparison.
• Line 135-136: It is believed that adding different alkyl chain changes the contact angle. Is the alkyl chain the only parameter to change the contact angle? Or the amount of alkyl chain is also important. For example, if we have two samples: (1) Adding 1 ml of 12C, (2) Adding 5 ml of 12C, do we get the same contact angle? If it has not been done, a control test like that is suggested.
• Fig. 2: A table with exact FEs for all products at different contact angles should be provided in the SI.
• Fig. 2e is confusing. It is better to have the same range for the y-axis.
• Line 177: for the stability test, the authors only showed current density. Product distribution is also important and should be mentioned.
• Supplementary Fig. 7: there are some missing products in these figures, including methane, formate, and acetate. Those products should be added to make sure the total FE is around 100%.
• Supplementary Fig. 11: the FEs should be added to show the stability of the system.
• While using physical vapor deposition method, it is important to mention the thickness as it affects surface facets and other morphologic characteristics. This information is missing.
• Cite a few papers to support line 219.
• Add the synthesis details for the CuAg catalyst.
• In addition to the CFD simulations, density functional theory (DFT) computations can leverage the quality of the work, especially by looking into the adsorption energy (or if possible, the reaction pathways) of *CO and *H on different Cu-xC catalysts.
Reviewer #3 (Remarks to the Author): The study investigates CO2RR using different modifyers. There are several works currently published on this topic, however I do find parts of this work interesting. But I have multiple comments: Please clarity what the authors mean when they put asterisk * on their molecules "*CO and *H". The typical convention is that * means that CO is adsorbed on the surface.   There is no discussion of Fig. 3 in the main body text. It seems labels are mixed with Fig 2. Please fix this, parts of the manuscript is very hard to follow when figure references are wrong.
"It is known that the mass transport of CO2 (local CO2 concentration) can affect the coverage of *CO2 (a precursor of *CO), *CO, and *H, which affects the reaction pathways toward ethylene and ethanol in Response: We thank the reviewer for the helpful comment very much. The SAM and Cu catalyst are stable in our experimental condition (1 M KOH (aq.), pH 14, −1.2 V vs. RHE). We have conducted 6.5 hours stability test in 1 M KOH (aq.) (pH 14) at −1.2 V vs. RHE and the durability of the alkanethiol monolayer (SAM) and Cu electrode was evaluated by XRD, XPS, TEM, CA and OH − electroadsorption measurement. The stability test shows a stable reduction current density with a stable product selectivity during durability test ( Supplementary Fig. 17). The morphology and exposed facets of Cu catalyst are not significantly changed by the SEM images, XRD profiles and OH − electroadsorption measurement (Supplementary Fig. 18,19,20). The XPS analysis also reveal similar Cu/S ratios on catalyst surface before and after the electrolysis ( Supplementary Fig.  21). The HRTEM displays the reservation of the alkanethiol layer on the Cu catalyst after reaction, which indicates the high electrochemical stability of hydrophobic layer ( Supplementary Fig. 22). Thus, the larger contact angle can be observed after electrolysis. (Supplementary Fig. 23). In this regard, the SAM and Cu catalyst are stable in our experimental condition (1 M KOH (aq.), pH 14, −1.2 V vs. RHE), which is in accordance with previous reports (Nano Energy, 2022, 92, 106784).
To make it clear for the readers, we have added associated discussions in the revised manuscript (Page 7).
"…The durability of the alkanethiol monolayer and Cu electrode was evaluated by XRD, XPS, TEM, contact angle measurement and OH − electroadsorption measurement. The 6.5 hours stability test at −1.2 V vs. RHE shows that a stable reduction current density and product selectivity are maintain after alkanethiol modification ( Supplementary Fig. 17). According to the SEM images, XRD profiles and OH − electroadsorption measurement, the morphology and dominant exposed facets of Cu catalyst are not significantly changed . XPS analysis ... In this regard, the exposed facet, morphology and hydrophobicity of the Cu catalyst and the thiol layer after electrolysis are relatively stable under experimental condition (1 M KOH (aq.), pH 14, −1.2 V vs. RHE), which is in accordance with the previous report 27,34 …" Specific Comments R1-5: The reported JV curves exhibit a clearly different behavior for samples coated with the SAMs. The authors perform most comparative analysis at a fixed potential vs RHE. This would be fair in terms of thermodynamics (the catalysts and reactants would face ~similar barriers), but would neglect important kinetic and dynamic environment properties such as OH coverage, pH etc. The authors are encouraged to perform similar analysis at fixed currents to offer a more complete vision on the proposed mechanisms.
Response: We thank the reviewer for the kind suggestion. We have added similar analysis at fixed currents (−100 mA cm −2 ) to obtain a more complete study of our catalyst. With the increasing hydrophobicity, the FE of ethylene increases gradually, while the FE of ethanol increases initially and then decreases gradually, indicating that increased hydrophobicity could promotes mass transport during the reduction. The production of ethanol is suppressed under superhydrophobicity due to the limited H2O transport.
We have added associated discussions in the Supplementary Information (Page 16, Supplementary Fig. 15). With the increasing hydrophobicity, the FE of ethylene increases gradually, while the FE of ethanol increases initially and then decreases gradually, indicating that increased hydrophobicity can promotes mass transport during the reduction. The production of ethanol is suppressed under super-hydrophobicity due to the limited H2O transport.

Response:
We thank the reviewer for the insightful comment. We have performed Electrochemical impedance spectroscopy (EIS) analysis to evaluate the influence of alkanethiols on the conductivity of modified electrodes, which can support the calculation of ECSA analysis ( Supplementary Fig. 27, Supplementary Table 4). All the spectra of EIS are characterized by a high-frequency feature which is associated with the charge transport, and a low-frequency arc is associated with the interfacial mass transport. The modeled equivalent circuit of EIS is inserted in Supplementary Fig. 27, where Rs is the electrolyte resistance, Rct is the charge transfer resistance, C is the double-layer capacitance and W is the Warburg impedance. The fitting procedures of all the samples are reported in Supplementary Table 4. The electrolyte resistance (Rs) of the Cu-xC electrodes is not as substantially perturbed as that of the bare Cu electrode. The charge resistance (Rct) and mass transport resistance (W) is increased with alkanethiols modification. The interfacial electrochemical double-layer capacitance (C) from EIS is also obtained, which can quantitatively evaluate the charge accumulation at the interface. The interfacial electrochemical double-layer capacitance (C) gradually decreases with the increasing of alkyl chain length on alkanethiols. Thus, the variation of charge resistance (Rct), mass transport resistance (W) and interfacial electrochemical double-layer capacitance (C) can be ascribed to the strong hydrophobicity induced less contact with the electrolyte after alkanethiol-modification, which is confirmed with the ECSA analysis.
We have added the ECSA analysis in the revised manuscript (Page 8) and Supplementary  "…The variation of charge resistance (Rct), mass transport resistance (W) and interfacial electrochemical double-layer capacitance (C) after alkanethiol-modification in Electrochemical impedance spectroscopy (EIS) analysis can be ascribed to the strong hydrophobicity that retarded the contact with the electrolyte after alkanethiol-modification, which is confirmed with the ECSA analysis (Supplementary Fig. 27,Supplementary Supplementary Fig. 27, where Rs is the electrolyte resistance, Rct is the charge transfer resistance, C is the double-layer capacitance and W is the Warburg impedance. The results of the fitting procedures for all the samples are reported in Supplementary Table 4. The electrolyte resistance (Rs) of the Cu-xC electrodes is not as substantially perturbed as that of the bare Cu electrode. The charge resistance (Rct) and mass transport resistance (W) is increased upon alkanethiols modification. The interfacial electrochemical double-layer capacitance (C) from EIS is also obtained, which can quantitatively evaluate the charge accumulation at the interface. The interfacial electrochemical double-layer capacitance (C) gradually decreases with increasing alkyl chain length on alkanethiols. Thus, the variation of charge resistance (Rct), mass transport resistance (W) and interfacial electrochemical double-layer capacitance (C) can be ascribed to the strong hydrophobicity that retarded the contact with the electrolyte after alkanethiol-modification, which is confirmed by the ECSA analysis. Response: Thanks to the reviewer for the valuable comment. It is true that there is a gap between our model and the actual catalyst. However, our simplified model is still representative to mimic the effect of wettability on the interfacial contact state and the local CO2/H2O ratio. As shown in scanning electron microscopy (SEM) images, Cu nano-islands are uniformly grown on the porous surface of carbon paper with an average size of around 1 µm ( Supplementary Fig. 2, 3). Therefore, the surface of the Cu electrode is indeed continuous in macroscale, while the structure of the Cu electrode can be approximated as catalyst islands with pores in microscale. To enable feasible CFD simulation, a simplified model is constructed as shown in Fig. 5a. The squares in the model are used to represent the catalysts, with the various contact angles setting. The gaps between the squares represents the porous surface of carbon paper between the real catalyst islands. The simulation results reveal the formation of continuous liquid layer on the hydrophilic catalyst (Cu) surface and CO2 needs diffuse through the thin liquid layer to reach the catalyst for reduction, which causes some hinder during gas transport (Fig. 5b). In comparison, the whole surface of high hydrophobicity (CA:156 o ) catalyst is mostly occupied by gas phase and formed a gas-solid interfaces. The hindrance of gas transport through thin liquid layer is eliminated, which would block H2O transport from electrolyte, simultaneously (Fig. 5d). In comparison, a gas-liquid-solid interface is formed on the lower hydrophobic catalyst (CA: 131 o ) with balanced wettability (Fig. 5c,d). The exposure of the three-phase interface could balance the gas and liquid mass transport, resulting in an optimized *CO/*H ratio for ethylene and ethanol conversion. The simulation results also proved that the spreading of gas and electrolyte on catalyst surface can be controlled by wettability, which is consistent with experimental data.

Supplementary
Thus, it is suitable to adopt this CFD model to understand the effect of wettability on the interfacial contact state and the local CO2/H2O ratio, which can reveal the mechanism of wettability variation influence on the ethylene and ethanol pathways. Thanks to the reviewers for the helpful suggestion. We have added the explanation of model establishment in the revised manuscript (Page13) to help readers better understand the effectiveness of the model. "…As shown in SEM images, Cu nano-islands are uniformly grown on the porous surface of carbon paper with an average size of around 1 µm ( Supplementary Fig. 2, 3). Therefore, the structure of the Cu electrode can be simplified as catalyst islands with pores. To enable the CFD simulation, a simplified model was constructed (Fig. 5a). The squares in the model are used to represent the catalysts with various contact angles. The gaps between the squares represent the pores between the real catalyst islands…" Specific Comments R1-8: The proposed picture in Fig 6 is nice but might be misleading. Have the authors proven that for the EtOH and C2H4 reactions CO2 is coming through the gas phase (and not dissolved in the electrolyte as mentioned in the intro) Response: Thanks to the reviewer for the valuable comment. It is true that CO2 has moderate solubility in aqueous electrolyte, so the reduced CO2 is ascribed to dissolved CO2 in the electrolyte in some previous reports. In our case, however, a gas-liquid-solid interface has been built to balance the CO2 and H2O transfer, which is essential for effective generate EtOH and C2H4 products. In comparison, less EtOH and C2H4 products can be detect from the hydrophilic catalyst, although the CO2 still can be dissolved in electrolyte layer nearby catalyst surface. Moreover, the results of CFD simulation and CLSM indicate that CO2 come through the gas phase on the hydrophobic-treated electrode, and some previous papers also have reported similar results (Science, 2020, 367, 661-666;Nat. Commun., 2021, 12, 136;Angew. Chem. Int. Ed., 2020, 59, 19095-19101;).
Specific Comments R1-9: Stability should be reported and sufficient to sustain the validity of all the studies presented herein. It is important to see if the SAM functionalization would affect stability.
Response: Thanks to the reviewer for the useful comment. We have conducted 6.5 hours stability test in 1 M KOH (aq.) at −1.2 V vs. RHE and the durability of the alkanethiol monolayer (SAM) and Cu electrode was evaluated by XRD, XPS, TEM, CA and OH − electroadsorption measurement. The stability test shows a stable reduction current density with a stable product selectivity ( Supplementary Fig. 17). The morphology and exposed facets of Cu catalyst are not significantly changed by the SEM images, XRD profiles and OH − electroadsorption measurement (Supplementary Fig. 18,19,20). The XPS analysis also reveal similar Cu/S ratios on catalyst surface before and after the electrolysis ( Supplementary Fig. 21). The HRTEM displays the reservation of the alkanethiol layer on the Cu catalyst after reaction, which indicates the high electrochemical stability of hydrophobic layer ( Supplementary Fig. 22). Thus, the larger contact angle can be observed after electrolysis. (Supplementary Fig. 23). In this regard, the SAM and Cu catalyst are stable in our experimental condition (1 M KOH (aq.), −1.2 V vs. RHE), which is in accordance with previous reports (Nano Energy, 2022, 92, 106784). The results of stability test are shown on the Page 7 in the revised manuscript.
"…The durability of the alkanethiol monolayer and Cu electrode was evaluated by XRD, XPS, TEM, contact angle measurement and OH − electroadsorption measurement. The 6.5 hours stability test at −1.2 V vs. RHE shows that a stable reduction current density and product selectivity are maintain after alkanethiol modification ( Supplementary Fig. 17). According to the SEM images, XRD profiles and OH − electroadsorption measurement, the morphology and dominant exposed facets of Cu catalyst are not significantly changed ( Supplementary Fig. 18-20). XPS analysis ... In this regard, the exposed facet, morphology and hydrophobicity of the Cu catalyst and the thiol layer after electrolysis are relatively stable under experimental condition (1 M KOH (aq.), pH 14, −1.2 V vs. RHE), which is in accordance with the previous report 27,34 …" Specific Comments R1-10: What is the single pass carbon utilization? Response: We thank the reviewer for the kind suggestion. The sample calculation of CO2 conversion is as follow (Joule, 2019, 3, 240-256): We have added the calculation of the single pass carbon utilization in the revised manuscript (Page 22) and the Supplementary Information (Page 41, Supplementary Table 2).
The calculation of CO2 conversion. The sample calculation of CO2 conversion is as follow 52 :

Response:
We thank the reviewer for the useful comment very much. We have added crosssectional SEM images of the Cu electrode. As shown in Supplementary Fig. 3, the area between the two yellow lines represents the catalyst layer. The thickness of the Cu electrode is around 1µm.
We have added the thickness of the Cu electrodes in the revised manuscript (Page 5) and the Supplementary Information (Page 4, Supplementary Fig. 3).
"…The cross-sectional SEM image also shows that the thickness of the Cu layer is around 1µm ( Supplementary Fig. 3 Response: The electrochemical conversion of carbon dioxide (CO2) into value-added multicarbon (C2+) products with high selectivity is still facing great challenges (Nature, 2020, 577, 509;Science, 2018, 360, 783;Science, 2019, 364, 350). *CO and *H are considered to be the key intermediates during the production of C2+ products (Joule, 2020(Joule, , 4, 1104Nat. Nanotechnol., 2019, 14, 1063. In general, local CO2/H2O concentration ratio that relevant with mass transport of CO2 and H2O can influence the surface coverage of *CO, and *H, which affects the reaction pathways toward C2+ products (Joule, 2020(Joule, , 4, 1104Nat. Commun., 2021, 12, 136). Modification of gas diffusion electrode (GDE) is a promising method to improve the transport of reactants and control the coverage of key intermediates (Science, 2020, 367, 661;Nat. Mater., 2019, 18, 1222Nat. Energy, 2021, 6, 1026. Although these long side chains containing -CH2-(Nat. Mater., 2019,18,[1222][1223][1224][1225][1226][1227] or -CF2-(Angew. Chem., 2020, 132, 160 -166;Nat. Commun., 2021, 12, 136) can induce strong hydrophobicity on catalyst surface to improve mass transfer, continuous control of wettability is still meeting great challenges. Moreover, although the C2+ selectivity could be enhanced by using hydrophobic polytetrafluoroethylene (PTFE) layers, it is difficult to identify and optimize the specific effect of hydrophobicity. Thus, the impact of interfacial wettability on the pathways of ethylene and ethanol is rarely understood, thus it is urgently desirable to develop a new approach to continuously tune the *CO/*H ratio by altering the interfacial wettability. This work proposes a direct and efficient interfacial modification protocol to continuously modulate the wettability of the catalyst. Through changing the substituents on alkanethiols, the balance of *CO and *H intermediate on surface can be effectively controlled. Therefore, this manuscript demonstrated the following novelty and new findings: 1) Continuous modulation of a hydrophilic interface to superhydrophobic; 2) Interfacial wettability control for variable *CO/*H ratio; 3) Tunable CO2/H2O ratio to optimize *CO/*H ratio for effective production of C2+ products. Thus, this work describes the design and realization of controllable equilibrium of *CO and *H hydrophobic via modifying alkanethiols with different alkyl chain lengths to reveal its contribution to ethylene and ethanol pathways.
We have added associated discussions on the page 4 in the revised manuscript.
"…To break the mass transport limitation and improve the selectivity of C2+ products, GDE can be modified to improve the transport of reactants and control the coverage of key intermediates 20,26-32 . Organic molecules (fluorosilane 33 , quaternary ammonium salt 29,30 ), polymers (polytetrafluoroethylene 20,28 ), ionic polymers (Nafion 26,31 ) can modulate the local concentrations of CO2 and H2O because their backbone chains containing -CH2-or -CF2-induce hydrophobicity. Although these long side chains can induce strong hydrophobicity on catalyst surface to improve mass transport effectively, continuous control of wettability is still meeting great challenges 20,28,33 . Moreover, although the C2+ selectivity can be enhanced by using hydrophobic polytetrafluoroethylene (PTFE) layers, it is difficult to identify and optimize the specific effect of hydrophobicity 27 . Thus, the impact of interfacial wettability on the pathways of products, especially ethylene and ethanol, is rarely understood. Therefore, it is urgently desirable to develop a new approach to continuously tune the *CO/*H ratio by altering the interfacial wettability…" Specific Comments R1-13: Language and typos (e.g., ECSA vs ECSA) should be fixed.

Response:
We thank the reviewer for the kind comment very much. We have thoroughly checked and corrected the language and typos in the manuscript (For example, on Page 8).
"…Additionally, the electrochemically active surface area (ECSA) also drops rapidly… Therefore, the lower ECSA and current density with the formation of alkanethiol layer ..." CO2 reduction requires water as a H source for hydrocarbon production (Science, 2020, 367, 661-666; Nat. Mater., 2019,18,[1222][1223][1224][1225][1226][1227]. Thus, the *H intermediate on surface is converted from the bulk solution (H2O). In this work, we used the water content to indirectly correlate *H coverage. The HER performance should not be affected by the external gas diffusion, but is determined by H2O availability and transport. Therefore, the HER performance of Cu catalysts with different wettability can be used to compare the H2O availability and *H transport, which is inspired by previous work (Science, 2020, 367, 661-666). We have revised the statement and added associated discussion in the revised manuscript to avoid misunderstandings. Thanks to the reviewer for the insightful comment. In the future, we are endeavored to develop advanced characterization methods to directly prove *H coverage, which would help us better understand the CO2RR process.
We have revised the statement and added associated discussion on page 12 in the revised manuscript.
"As another key intermediate, the *H intermediate on surface is converted from the bulk solution, which is affected by H2O transport. Unfortunately, there is no direct experimental method to observe *H coverage on Cu so far, because the adsorption of *H on Cu is too weak. At present, *H coverage can be investigated indirectly through * CO coverage or H2O content 18,46-49 . *H and *CO occupy most of the Cu surface sites, resulting in direct competition between *H and *CO for surface sites. When the surface maintains a high *CO coverage, the corresponding *H coverage will decrease. In order to elucidate the effect of H2O transport on the ethylene and ethanol reaction pathways, the HER performance of Cu catalysts with different wettability for various electrolytes was compared (Fig. 4a, b). The HER performance should not be affected by the external gas diffusion, but is determined by H2O availability and transport 26 . Therefore, stronger hydrophilicity can benefit the HER due to more efficient H2O transport."

Specific Comments R2-2: The role of wettability on *CO and *H coverage is shown (if we accept the discussion for the *H coverage for now), but still, it is not clear why and how these surface coverages affect the reaction pathway towards ethylene and ethanol? That is a key question which is stated in the introduction but not fully answered in this paper. To be more specific, why we have ethanol, and no other alcohols or oxygenates like methanol, propanol or acetate? Why this surface wettability affects ethylene and no other hydrocarbons like methane?
Response: Thanks to the reviewer for the valuable comment. *H and *CO occupy most of the Cu surface sites, so they are in direct competition with each other for surface sites. When the surface maintains a high *CO coverage, the corresponding *H coverage will decrease (J. Further, we have supplemented the Faradaic efficiency data and analysis of the all products. There is almost no methanol product on copper (Nat. Energy, 2019, 4, 732-745), while methane and formate products can be observed. The variation trend of methane Faradaic efficiency is consistent with that of ethylene. The Faradaic efficiencies of formate are relatively higher at low potentials, but lower at high potentials. We also observed that the variation trend of propanol Faradaic efficiency is similar as that of C2 products, which is because C2 product is the key intermediate of propanol pathway (Nat. Commun., 2021, 12, 1580.). With the decreasing of CO, the Faradaic efficiency of propanol decreases significantly. However, very few acetates were identified in our experiment, which is due to the different formation mechanism of acetic acid with other C2 products (ethylene and ethanol). The Faradaic efficiency of acetate in CORR is obviously higher than that of in CO2RR. The rich *CO coverage on Cu surface in CORR could stabilize ethenone (as a key acetate path intermediate) and inhibit the hydrogen evolution reaction, thus substantially promote acetate formation. A highly alkaline environment (a high pH value) near the electrode-electrolyte interface in CORR can also promote the formation of acetate. Thus, the activity and selectivity of producing acetate were low, and acetate is not the main product in CO2RR, which also have been reported previously (Nat. Catal., 2022, 5, 251-258; Nat. Catal.,2019, 2, 423-430; Nat. Catal., 2018, 1, 748-755).
We have supplemented the Faradaic efficiency data and analysis of the all products in the Supplementary Information (Page 11, Supplementary Fig. 10). Further, we also have added associated discussions on page 7 and page 12 in revised manuscript.
"…*H and *CO occupy most of the Cu surface sites, resulting in direct competition between *H and *CO for surface sites. When the surface maintains a high *CO coverage, the corresponding *H coverage will decrease…" "…Moreover, the variation trend of methane Faradaic efficiency is consistent with that of ethylene, while the variation trend of propanol Faradaic efficiency is similar as that of C2 products ( Supplementary Fig. 10 The variation trend of methane Faradaic efficiency is consistent with that of ethylene. The Faradaic efficiencies of formate are relatively higher at low potentials, but lower at high potentials. The variation trend of propanol Faradaic efficiency is similar to that of C2 products, because C2 product is the key intermediate of propanol pathway.
Specific Comments R2-3: In this work it is shown that, the higher *CO coverage is expected when the surface is more hydrophobic. And it's shown that the more hydrophobic surface such as Cu-12C is more selective to produce ethanol compared to ethylene. But then, why in CORR experiment we see more ethylene than ethanol compared to the CO2RR experiment? Should not it be the other way around? I would expect a higher *CO surface coverage in CORR compared to that in CO2RR, and as it's shown via ATR_SERIRAS test the higher *CO is the consequence of the more hydrophobic surface and logically we should expect more ethanol.

Response:
We thank the reviewer for the helpful comment very much. The selectivity of ethanol and ethylene first increases and then decreases (like a volcano) with increasing *CO coverage (Nat. Catal., 2020, 3, 75-82; Nat. Catal., 2019, 2, 1124-1131). As we mention above, the generation of ethanol is in relevance with *CO coverage and *H coverage simultaneously, both of which need to be balanced to achieve high selectivity of ethanol. In our CORR experiment, the direct introduction of CO could lead to excessive *CO coverage, but the *H coverage still be preserved at a certain level as CO2RR experiment. Thanks for the reviewer's kind suggestion, and we reduce the CO feeding concentration in updated CORR experiment, and more ethanol in CORR can be obtained, which can better support our proposed mechanism.
The updated CORR result and discussion is shown in page 9 and 10 in revised manuscript.
"…Like in the CO2RR, the direct coupling between two *CO species in the CORR is widely accepted as the major C-C coupling mechanism. Moreover, *CO coverage is determined by the local concentration of CO near catalyst. Thus, the coverage of key intermediate *CO can be improved by directly using CO as reactants. When the Cu catalyst has similar contact angle, the promotion of ethanol production is more obviously than that of ethylene during CORR. In comparison, the similar phenomenon can be observed experiments on CuAg catalyst, which can generate more CO during performing CO2RR ( Supplementary Fig. 32). Therefore, increasing *CO coverage will benefit to ethanol generation." Fig. 3 │Effect of controllable wettability on *CO coverage via CO2 mass transport. a, Top row: schematics display the CO2RR configuration with (right column) or without (left column) alkanethiol modification. The red dotted box is the simulation area. Second row: comparison of modeled gas availability along the catalyst surface with (right column) or without (left column) alkanethiol modification. Gas availability dramatically increases via alkanethiol-derived hydrophobic environment. b, Local CO2 concentration versus time during the CO2RR of 100 mA cm −2 of modeled Cu, Cu-12C and Cu-18C interfacial environments, respectively. A stronger hydrophobicity indicates a faster CO2 mass transport. c, Comparison of ethylene and ethanol Faradaic efficiencies for CO2RR versus CORR on Cu, which indicates the improvement of *CO coverage is more favorable for the ethanol pathway than for ethylene. In-situ ATR-SEIRAS spectra of d, Cu and (e) Cu-12C, revealing that higher *CO coverage can be maintained on Cu electrodes after hydrophobic treatment. Wherein the stretching band at ~2070 cm −1 corresponds to the stretching band of COL on Cu surface.

Specific Comments R2-4:
The potential impact on the reaction pathway is important. By saying potential, we mean the potential on the surface of the catalyst. However, ignoring the iR potential loss cannot give us a realistic picture of what potential exists at the cathode. Therefore, the whole discussion in this manuscript can go under question that perhaps all observations are due to the potential shift, and that's why we see different product distribution. EIS test must be performed to measure the impedance of the system and also to elaborate more on the charge transfer resistance for each catalyst.
Response: Thanks to the reviewer for the valuable suggestion. We have supplemented the relevance between Faradaic efficiencies of ethanol and various cathode potential after iR correction in Supplementary Fig. 14. Comparing the profiles of Cu, Cu-12C and Cu-18C (Supplementary Fig. 14a and Supplementary Fig. 14b), a similar tendency can be observed before and after iR correction. Therefore, the different product distribution is not due to the potential shift.
Further, we have performed electrochemical impedance spectroscopy (EIS) analysis to evaluate the influence of alkanethiols on the conductivity of modified electrodes, which can support the calculation of ECSA analysis ( Supplementary Fig. 27, Supplementary Table 4). All the spectra of EIS are characterized by a high-frequency feature which is associated with the charge transport, and a low-frequency arc associated with the interfacial mass transport. The modeled equivalent circuit of EIS is inserted in Supplementary Fig. 27, where Rs is the electrolyte resistance, Rct is the charge transfer resistance, C is the double-layer capacitance and W is the Warburg impedance. The fitting procedures of all the samples are reported in Supplementary Table 4. The electrolyte resistance (Rs) of the Cu-xC electrodes is not as substantially perturbed as that of the bare Cu electrode. The charge resistance (Rct) and mass transport resistance (W) is increased with alkanethiols modification. The interfacial electrochemical double-layer capacitance (C) from EIS is also obtained, which can quantitatively evaluate the charge accumulation at the interface. The interfacial electrochemical double-layer capacitance (C) gradually decreases with the increasing alkyl chain length on alkanethiols. Thus, the variation of charge resistance (Rct), mass transport resistance (W) and interfacial electrochemical double-layer capacitance (C) can be ascribed to the strong hydrophobicity induced less contact with the electrolyte after alkanethiol-modification.
We have added associated discussions about iR correction in Supplementary Information (Page 15, Supplementary Fig. 14). Moreover, the EIS analysis is added in the revised manuscript (Page 8) and the Supplementary  The relevance between Faradaic efficiencies of ethanol and various cathode potential before and after iR correction is shown in Supplementary Fig. 14. Comparing with the profiles of Cu, Cu-12C and Cu-18 C (Supplementary Fig. 14a and Supplementary Fig. 14b), a similar tendency can be observed before and after iR correction. Therefore, the products distribution and reaction efficiency are related with the hydrophobicity of catalyst. Supplementary Fig. 27 | EIS spectra of the Cu, Cu-12C and Cu-18C electrodes. The EIS is modeled by equivalent circuit inserted in Supplementary Fig. 27, where Rs is the electrolyte resistance, Rct is the charge transfer resistance, C is the double-layer capacitance and W is the Warburg impedance. The results of the fitting procedures for all the samples are reported in Supplementary Table 4. The electrolyte resistance (Rs) of the Cu-xC electrodes is not as substantially perturbed as that of the bare Cu electrode. The charge resistance (Rct) and mass transport resistance (W) is increased upon alkanethiols modification. The interfacial electrochemical double-layer capacitance (C) from EIS is also obtained, which can quantitatively evaluate the charge accumulation at the interface. The interfacial electrochemical double-layer capacitance (C) gradually decreases with increasing alkyl chain length on alkanethiols. Thus, the variation of charge resistance (Rct), mass transport resistance (W) and interfacial electrochemical double-layer capacitance (C) can be ascribed to the strong hydrophobicity that retarded the contact with the electrolyte after alkanethiol-modification, which is confirmed by the ECSA analysis. Electrochemical impedance spectroscopy (EIS). EIS measurements were performed in a flow cell at room temperature with Autolab electrochemical workstation. Ni foam and saturated Hg/HgO electrode was used as the counter electrode (CE) and the reference electrode (RE), respectively. The EIS measurements were carried out in 1 M KOH solution at open circuit potential (OCP). The impedance spectra were recorded with an amplitude of 10 mV from 0.01 to 100 kHz. The data obtained from the EIS measurements were fitted by the Zview software (Version 3.1, Scribner Associates, USA).

Specific Comments R2-5:
In the introduction, one of the main challenges in the field is discussed as ethylene/ethanol selectivity control, however, throughout the manuscript, ethylene is replaced by C2+ products invariantly which makes the story difficult to follow and to draw a clear conclusion.
Response: Thanks to the reviewer for the valuable suggestion. We have checked the manuscript throughout and make some suitable revisions to avoid the misunderstanding. For example (Page 11 and Page 17): "The local CO2 concentration can further influence the coverage of *CO, which affects the reaction pathways toward ethylene and ethanol…" "…Therefore, the reaction pathways of ethylene and ethanol can be tuned by kinetic-controlled *CO/*H ratio through controllable wettability…" Specific Comments R2-6: To further support the hypothesis that the surface wettability is the only parameter affecting the selectivity (at least in this study), the authors need to repeat the experiment with a different control sample with a given hydrophobicity. For example, applying PTFE on the surface, measuring the contact angel, running the CO2RR, and monitoring the ethanol/ethylene ratio.
Response: Thanks to the reviewer for the valuable comment. We have repeated the CO2RR experiment with PTFE modification to measure the effect of wettability on the ethanol/ethylene ratio. As shown in Supplementary Fig. 16, only a small amount of PTFE is needed to induce suprahydrophobicity on catalyst surface, which leads to great challenges in the continuous control of wettability. Moreover, it is difficult to differentiate the specific effect of wettability on ethanol/ethylene ratio over other factors by introducing PTFE layers. Thus, the impact of interfacial wettability on the pathways of products, especially ethylene and ethanol, is difficultly understood by using PTFE.
The ethanol to ethylene ratio on PTFE modified Cu electrodes has been added in Supplementary  Information (Page 17, Supplementary Fig. 16). Fig. 16 | Ethanol to ethylene ratio on PTFE modified Cu electrodes in 1 M KOH (aq.) at −1.2 V versus the RHE with various contact angles. As shown in Supplementary Fig. 16, only a small amount of PTFE is needed to induce suprahydrophobicity on catalyst surface, which leads to great challenges in the continuous control of wettability. Moreover, it is difficult to differentiate the specific effect of wettability on ethanol/ethylene ratio over other factors by introducing PTFE layers. Thus, the impact of interfacial wettability on the pathways of products, especially ethylene and ethanol, is difficultly understood by using PTFE.

Supplementary
Specific Comments R2-7: Line 120: nano-features are not visible in Supplementary Fig. 2. The scale bar in SEM images is in micro scale.
Response: Thanks to the reviewer for the kind comment. We have revised this mistake on page 3 in the Supplementary Information (Supplementary Fig. 2). Fig. 2 │ SEM images of Cu-12C. Cu nano-islands are uniformly-grown on the carbon paper surface with an average size of around 1 µm.
Response: Thanks to the reviewer for the helpful suggestion. We have rewritten the caption of Figure 3c to label the results of Cu instead of Cu-12C on page 9 in revised manuscript. Fig. 3 │Effect of controllable wettability on *CO coverage via CO2 mass transport. a, Top row: schematics display the CO2RR configuration with (right column) or without (left column) alkanethiol modification. The red dotted box is the simulation area. Second row: comparison of modeled gas availability along the catalyst surface with (right column) or without (left column) alkanethiol modification. Gas availability dramatically increases via alkanethiol-derived hydrophobic environment. b, Local CO2 concentration versus time during the CO2RR of 100 mA cm -2 of modeled Cu, Cu-12C and Cu-18C interfacial environments, respectively. A stronger the hydrophobicity indicates a faster CO2 mass transport. c, Comparison of ethylene and ethanol Faradaic efficiency enhancement values for CO2RR versus CORR on Cu, which indicates the improvement of *CO coverage is more favorable for the ethanol pathway than for ethylene. In-situ ATR-SEIRAS spectra of d, Cu and (e) Cu-12C, revealing that higher *CO coverage can be maintained on Cu electrodes after hydrophobic treatment. Wherein the stretching band at ~2070 cm -1 corresponds to the stretching band of COL on Cu surface. Specific Comments R2-9: There are several typos, grammatical mistakes, and difficult sentences to read, even wrong figure numbers (most Fig. 2's must be Fig. 3's). The manuscript needs to be proofread again before resubmission.
Response: Thanks to the reviewer for the kind suggestion. We have checked up and revised the resubmitted manuscript. For example, on page 9 and page 10 in revised manuscript.
"… Our data (Fig. 2) also imply that the *CO/*H ratio could be controlled by tuning the local CO2/H2O ratio through changing wettability of the catalyst…The models with and without the alkanethiol modification layer were established to quantity dissolved CO2 in solution (Fig. 3a)…" "… To investigate the local CO2 concentration of different interfacial wettability, control samples were prepared with different gas-liquid-solid contact, enabling in situ measurements with fluorescence electrochemical spectroscopy (FES) at 100 mA cm -2 chronopotentiometry (Fig.  3b)…" "…The hydrophobic treatment also can promote CO diffusion in CO reduction reaction. Thus, it can be excluded that the limited CO2 mass transport at the hydrophilic Cu electrode is entirely due to the neutralization of CO2 with the electrolyte (Fig. 3c, Supplementary Fig. 23) …" "…The local CO2 concentration can further influence the coverage of *CO, which affects the reaction pathways toward C2+ products. Thus, in-situ ATR-SEIRAS was employed to further evaluate the impact of interfacial wettability on the adsorption of intermediates (Fig. 3d, e) ..." Specific Comments R2-10: Line 129-130: a table of deconvoluted peaks for XPS results could be provided in SI. Also, a survey can be added.
Response: Thanks to the reviewer for the valuable comment. The deconvoluted peaks for XPS have been added as Supplementary Fig. 5, 6. The Cu catalyst consists of Cu + /Cu 0 (932.5/952.4 eV) and Cu 2+ (934.8/954.6 eV and 943.6/962.7 eV) (Fig. 1d, Supplementary Fig. 5). For a better comparison, the surface of bare Cu sample was analyzed in XPS with Ar etching. After Ar etching, the bare Cu sample presents Cu 2p peaks at 932.5 eV and 952.4 eV, corresponding to Cu + /Cu 0 species (Supplementary Fig. 6a). However, the binding energy of Cu 0 (932.6 eV) and Cu + (952.5 eV) in Cu 2p XPS spectra is too close to be identified, so we have conducted Auger electron spectroscopy (AES) of Cu LMM. The Cu 2p XPS spectra and Auger electron spectroscopy (AES) of Cu LMM both indicate that Cu mainly consists of Cu 0 , Cu + and Cu 2+ , while the Cu catalyst after Ar etching mainly consists of Cu 0 .
We have added associated discussions in the revised manuscript (Page 6) and Supplementary Information (Page 6-7, Supplementary Fig. 5,6).
"…The Cu-S coordinate bonds were formed via alkanethiolation as illustrated by X-ray photoelectron spectroscopy (XPS) of Cu 2p and Auger electron spectroscopy (AES) of Cu LMM.
Before modification, the Cu catalyst consists of Cu + /Cu 0 (932.5/952.4 eV) and Cu 2+ (934.8/954.6 eV and 943.6/962.7 eV) (Fig. 1d, Supplementary Fig. 5) 35 . For a better comparison, the surface of the bare Cu sample was analyzed in XPS with Ar etching. After Ar etching, the bare Cu sample presents Cu 2p peaks at 932.5 eV and 952.4 eV, corresponding to Cu + /Cu 0 species ( Supplementary  Fig. 6a). The AES of Cu LMM further indicates that the bare Cu sample mainly consists of Cu 0 (918.6 eV) after Ar etching ( Supplementary Fig. 6b)  Response: Thanks to the reviewer for the helpful comment very much. As the reviewer speculated, the Cu surface can be rapidly oxidized to Cu 2+ in the atmosphere, which introduced a little amount of Cu 2+ on catalyst surface. To eliminate misunderstanding and compare more accurately, we have etched the surface of the bare Cu sample by Ar before XPS. After Ar etching, the bare Cu sample presents Cu 2p peaks at 932.5 eV and 952.4 eV, corresponding to Cu + /Cu 0 species ( Supplementary  Fig. 6a). The AES of Cu LMM further indicates that the bare Cu sample mainly consists of Cu 0 (918.6 eV) after Ar etching (Supplementary Fig. 6b).
We have added XPS and Auger Cu LMM spectrum in the revised manuscript (Page 6) and Supplementary Information (Page 7, Supplementary Fig. 6).
"…The Cu-S coordinate bonds were formed via alkanethiolation as illustrated by X-ray photoelectron spectroscopy (XPS) of Cu 2p and Auger electron spectroscopy (AES) of Cu LMM. Before modification, the Cu catalyst consists of Cu + /Cu 0 (932.5/952.4 eV) and Cu 2+ (934.8/954.6 eV and 943.6/962.7 eV) (Fig. 1d, Supplementary Fig. 5) 35 . For a better comparison, the surface of the bare Cu sample was analyzed in XPS with Ar etching. After Ar etching, the bare Cu sample presents Cu 2p peaks at 932.5 eV and 952.4 eV, corresponding to Cu + /Cu 0 species ( Supplementary  Fig. 6a). The AES of Cu LMM further indicates that the bare Cu sample mainly consists of Cu 0 (918.6 eV) after Ar etching ( Supplementary Fig. 6b)  Response: Thanks to the reviewer for the valuable comment. We have conducted the control test according to the suggestions of the reviewer, and we obtained the same contact angle (1 mL 12C vs. 5 mL 12C). In fact, the formation of the thiol layer proceeds in pure thiol liquid to ensure that thiol molecules reach saturated adsorption on the copper surface. After this, the electrode was washed by ethyl acetate to remove excessive thiol. Either 1 mL or 5 mL 12C can reach saturated monolayer adsorption on the Cu surface, resulting in almost the same contact angle.
We have added controlled experiment in the Supplementary Information (Page 8, Supplementary Fig. 7). Fig. 7 │ Contact angle of the Cu electrode modified with different amount of 1dodecanethiol (12C). The formation of the thiol layer was performed in pure thiol liquid to ensure that thiol molecules reach saturated adsorption on the copper surface. After this, the electrode was washed by ethyl acetate to remove excessive thiol. Either 1 mL or 5 mL 12C can reach saturated monolayer adsorption on the Cu surface, resulting in almost the same contact angle.

Supplementary
Specific Comments R2-13: Fig. 2 Specific Comments R2-14: Fig. 2e is confusing. It is better to have the same range for the y-axis.
Response: Thanks to the reviewer for kind suggestion. We have revised the y-axis in the same range in the revised manuscript (Page 9, Fig 3e).

Response:
We thank the reviewer for valuable suggestion. The reduction current density and product selectivity are kept stable after alkanethiol-modification. We have added product distribution of the stability test in Supplementary Information (Page 18, Supplementary Fig. 17).
Supplementary Fig. 17 | Stability test of Cu-12C for CO2 electrolysis over 6.5 h (390 min) in 1 M KOH (aq.) at −1.2 V versus the RHE. The reduction current density and product selectivity are kept stable after alkanethiol-modification.
Specific Comments R2-16: Supplementary Fig. 7: there are some missing products in these figures, including methane, formate, and acetate. Those products should be added to make sure the total FE is around 100%.

Response:
We thank the reviewer for useful suggestion. We have supplemented the Faradaic efficiency of the all products in the Supplementary Information (Page 11, Supplementary Fig. 10). Specific Comments R2-17: Supplementary Fig. 10: the caption is incorrect. Cu-xC, not Cu-12C.

Response:
We thank the reviewer for useful suggestion. We have corrected this mistake in the Supplementary Information (Page 14, Supplementary Fig. 13).
Supplementary Fig. 13 | Ethanol-to-ethylene ratio of Cu-xC at various potentials. The ethanolto-ethylene ratios increase then subsequently drop with the increasing of contact angle at various potentials.
Specific Comments R2-18: Supplementary Fig. 11: the FEs should be added to show the stability of the system.

Response:
We thank the reviewer for kind suggestion. The reduction current density and product selectivity are kept stable after alkanethiol-modification. We have added product distribution of the stability test in the Supplementary Information (Page 18, Supplementary Fig. 17).
Supplementary Fig. 17 | Stability test of Cu-12C for CO2 electrolysis over 6.5 h (390 min) in 1 M KOH (aq.) at −1.2 V versus the RHE. The reduction current density and product selectivity are kept stable after alkanethiol-modification. Specific Comments R2-19: Supplementary Fig. 19: is the geometric surface area, correct? Almost 8m 2 /g?

Response:
Thanks to the reviewer for kind suggestion. The geometric surface area in Supplementary Fig. 26 should be the Brunauer-Emmett-Teller specific surface areas (SBET). It is difficult to obtain enough BET data for samples prepared by sputtering method. Thus, the copper powder with and without alkanethiol treatment were used to verify the influence of alkanethiol modification on Brunauer-Emmett-Teller specific surface area. We have revised this mistake in the Supplementary Information (Page 27, Supplementary Fig. 26).

Specific Comments R2-20: While using physical vapor deposition method, it is important to mention the thickness as it affects surface facets and other morphologic characteristics. This information is missing.
Response: We thank the reviewer for the helpful suggestion. We have added cross-sectional SEM images of the Cu electrode. The area between the two yellow lines represents the catalyst layer. The thickness of the Cu layer is around 1µm.
We have added the thickness of the Cu electrodes in the revised manuscript (Page 5) and the Supplementary Information (Page 4, Supplementary Fig. 3).
"…The cross-sectional SEM image also shows that the thickness of the Cu layer is around 1µm (Supplementary Fig. 3

Specific Comments R2-21: Cite a few papers to support line 219.
Response: Thanks to reviewer for the kind suggestion. We have cited associated papers to support line 219 on page 9 in revised manuscript.
"It is known that the mass transport of CO2 (local CO2 concentration) can affect the coverage of *CO2 (a precursor of *CO), *CO, and *H, which affects the reaction pathways toward ethylene and ethanol in further 5,20,42 Supplementary Fig. 37, 38).
"…Compared with the classical modulation of active site, the novelty of our method is that the CO2RR is completely controlled by kinetics (mass transport) through controllable interfacial wettability ( Supplementary Fig. 37, 38)." The adsorption energy of *CO and *H on Cu-xC catalysts with different alkyl chain lengths is similar. Further, the adsorption of *H on Cu(111) is much weaker than that of *CO. Fcc-1 Fcc-2 Fcc-2 Fcc-1 Density Functional Theory (DFT) Methods. The theoretical calculation was conducted by Vienna ab initio simulation package (VASP) with the BEEF-vdW exchange-correlation functional. As for the simulation models, we built five-layer Cu(111)-(4×4) slabs with a Butyl Mercaptan and 1-dodecanethiol molecular to compare the effect of different lengths of thiols on the reaction mechanism. Three bottom layers were fixed while the upper layers were relaxed in these models. The periodic interactions between repeated slabs were minimized by the vacuum space of at least 15 Å. As for the calculation settings, the cut-off energy is 400 eV. The interactions between the atomic cores and electrons were described by the projector augmented wave (PAW) method. All structures were optimized until the force on each atom has been less than 0.02 eV/Å. A (3×3×1) k-point grid were employed for the simulation models. The free energy of the *CO and *H (ΔGads) was calculated as follows: ΔG *CO = G *CO -G CO(gas) -G surface ΔG *H = G *H -1/2G H2(gas) -G surface General Comments R3: The study investigates CO2RR using different modifyers. There are several works currently published on this topic, however I do find parts of this work interesting. But I have multiple comments: Response: We appreciate the reviewer very much for the valuable comments. We have carefully revised the manuscript in accordance with the reviewer's comments.
Specific Comments R3-1: Please clarity what the authors mean when they put asterisk * on their molecules "*CO and *H". The typical convention is that * means that CO is adsorbed on the surface.
Response: Thanks to the reviewer for helpful suggestion. * means that CO is adsorbed on the surface. We have clarified * on page 2 and page 3 in our revised manuscript.
"Abstract: …This paper describes the design and realization of controllable equilibrium of *CO and *H (* denotes the adsorbed species on the surface) hydrophobic via modifying alkanethiols with different alkyl chain lengths to reveal its contribution to ethylene and ethanol pathways…" "… *CO and *H (* denotes the adsorbed species on the surface) are considered to be the key intermediates during producing C2+ products 4-6 . Previously, *CO coverage has been widely accepted as one crucial factor on selective produce ethylene 4,5 …" Specific Comments R3-2: The discussion of, or the framing of, *CO/*H feels a bit out of place. As a GDE does not change the binding energies of *CO or *H, but it rather changes the diffusion or chemical potential at the interface of the species, resulting in an improved performance of a GDE over a normal aquoues test. I believe the authors should be more clear on this point -as now this is mixed together. One example is: "This work describes the design and realization of a strategy that continuously tunes the controllable equilibrium of *CO and *H by altering the interfacial wettability via alkyl thiols with different alkyl chain lengths." Response: Thanks to the reviewer for valuable suggestion very much. We strongly agree with the reviewer that a GDE does not change the binding energies of *CO or *H. We apologize that we did not distinguish thermodynamic and kinetic factors well in the manuscript, which leads to misleading. In fact, catalytic reaction is related to both thermodynamics and kinetics. When the mass transfer rate is fast, the reaction rate is determined by thermodynamics. When the mass transfer is limited, the reaction rate is determined by kinetics. In this work, the catalytic reaction is controlled by kinetics (mass transfer) without changing thermodynamics due to the sluggish mass transfer. Specifically, we strove to increase the reaction rate of one step without strongly modulating the others, by judiciously increasing the coverage of one key intermediate, yet not interfering with the electronic structure (hence binding strength) of Cu. As in previous report, some molecular-metal composite was developed to generates a high concentration of the key early intermediate, yet does not modulate the metallic active sites germane to the crucial C-C coupling step, which is essentially similar to our work, but through a different method (Nat. Catal., 2020,3,[75][76][77][78][79][80][81][82]).
An independent means of controlling CO coverage is by changing the CO partial pressure or local CO concentration ([CO]), where the local CO concentration correlates with pCO by Henry's law. At equilibrium, the surface coverage of CO (θCO) is directly proportional to the local CO partial pressure, as given by the following equation (Nat. Catal., 2019,22,[1124][1125][1126][1127][1128][1129][1130][1131]: Where θ is the coverage of free surface sites, ECO is the CO adsorption energy on the surface, R is the ideal gas constant and T is the temperature. The coverage of intermediate (θCO) is a function of CO adsorption energy (ECO) and local CO concentration ([CO]). To vary the CO coverage, changing the local concentration of CO at catalysts layer rather than CO adsorption energy is therefore a promising approach. Compared with the classical modulation of active site, the novelty of our method is that the CO2RR is completely controlled by kinetics (mass transport) through controllable interfacial wettability, which has not been reported so far.
Our method is not as direct as changing the adsorption energy. The classical design of active site is still very important, but our work provides a new approach on catalyst design based on the mass transport for future industrial application.
We have added associated explanations on page 4 and page 16 in revised manuscript.
"…Further, the reaction pathways on catalyst surfaces with different wettability can be understood (Fig. 6c). The surface coverage of adsorbed *COx (θCOx, x=1 or 2) on the catalyst is proportional to the local concentration of COx ([COx]), which is given as Equation 1 5,42 : where θ is the coverage of free surface sites, ECOx is the COx adsorption energy on the surface, R is the ideal gas constant and T is the temperature. The coverage of intermediate (θCOx) is a function of CO adsorption energy (ECOx) and local CO concentration ([COx]). To vary the *CO coverage, changing the local concentration of CO at catalysts layer rather than *CO adsorption energy is therefore a promising approach. Compared with the classical modulation of active site, the novelty of our method is that the CO2RR is completely controlled by kinetics (mass transport) through controllable interfacial wettability ( Supplementary Fig. 37, 38)…" "This work describes the design and realization of tunable interfacial wettability through using different alkyl thiols. Then, the local concentration of CO2 and H2O can be modulated by changing CO2 and H2O transport through different interface wettability, which can achieve an optimized equilibrium of kinetic-controlled *CO and *H in a controllable manner…" Specific Comments R3-3: There are several sentences/statements with misleading references which needs to be corrected. Particularly in the introduction, I give some examples here: "*CO and *H are considered to be the key intermediates during producing C2+ products" Indeed *CO and *H are key intermediates for a predictive scheme for CO2RR as first shown by Bagger et al. https://doi.org/10.1002/cphc.201700736 "As the only metal with a negative adsorption energy for *CO but a positive adsorption energy for *H, copper (Cu) presents a unique ability to produce C2+ products 9,10 . However, multiple products have been detected on Cu surfaces resulting in poor product selectivity for Cu 11 ." Please refer to the original and first publications by Bagger et al. https://doi.org/10.1002/cphc.201700736 andhttps://doi.org/10.1021/acscatal.9b01899 "Previous investigations have found that the coverage of *CO and *H on Cu surface plays a critical role on determining selectivity of C2+ products 12 ." Ref 12: Energy Environ. Sci. 5, 7050-7059 (2012). ref 12 does not discuss *CO and *H on the Cu plays a critical role.

Response:
We thank the reviewer for helpful suggestion. Bagger et al. have first proposed four non-coupled binding energies of intermediates (H*, COOH*, CO*, and CH3O*) as descriptors, for predicting the product distribution in CO2 electroreduction. This work helps us understand what surface properties determine the main product of CO2 reduction (A. Bagger et al., ChemPhysChem 2017, 18, 3266-3273). Bagger et al. also originally found that two descriptors, △ECO* and △EH*, could explain the further reduction to products beyond CO* (A. Bagger et al., Catal. Today 2017, 288, 74-78). Further, they proposed that the Cu catalyst is unique for CO2 reduction reactions, as compared with other metals, because of its ability to produce a wide range of hydrocarbon and oxygenated products (A. Bagger et al., ACS Catal., 2019, 9, 7894−7899). Cu is the only metal catalyst to give products beyond CO, which is because Cu binds CO* while not having Hupd (A. Bagger et al., ChemPhysChem 2017, 18, 3266-3273). Moreover, they also proposed that the analysis for the ratio of diffusion between CO2 and H2O helping to shed light on a likely mechanism behind the changes in selectivity for functionalized Cu surfaces (A. Bagger et al., ACS Catal., 2022, 12, 15737−15749). Importantly, they further expanded these studies to bond the C atom of CO2 with other valuable heteroatoms (e.g., N), which is an alternative strategy to produce value-added products and highly beneficial for expanding the application of CO2RR (A. Bagger et al., ACS Catal., 2023, 13, 1926−1933. A series of innovative work by Bagger et al. has given us a deeper understanding of the critical role of key intermediates on the Cu for C2+ product production.
We have corrected the misleading references on page 3 in revised manuscript.
"…*CO (* denotes the adsorbed species on the surface) and *H are considered to be the key intermediates during producing C2+ products 4 …" "…As the only metal with a negative adsorption energy for *CO but a positive adsorption energy for *H, copper (Cu) presents a unique ability to produce C2+ products 4,9 . However, multiple products hav e been detected on Cu surfaces resulting in poor product selectivity for Cu 9,10 …" "…Previous investigations have found that the coverage of *CO and *H on Cu surface plays a critical role on determining selectivity of C2+ products 4,11 …" Response: Thanks to the reviewer for valuable comment. As the reviewer commented, the electrolyte KOH will be neutralized by CO2 after a long time of reaction, resulting in the kinetic degradation. In the flow cell, the flowing of electrolyte can refresh the local pH on electrode surface. Moreover, the electrolyte needs to be renewed after a long operation. For our work, CO2 comes through the gas phase after the hydrophobic treatment. Compared with the effect of mass transport, the effect of electrolyte acidification is relatively small, which is consistent with previous reports (Science, 2020, 367, 661-666;Nat. Commun., 2021, 12, 136). Therefore, mass transport is still the main influencing factor in our work, and the acidification of the electrolyte does not affect the conclusion of our work.  Table 5). The lower ECSA and current density after modified alkanethiol layer can be ascribe to the reduced contact with the aqueous electrolyte ( Fig. 2f, Supplementary Fig. 18, 19). However, Cu-12C (93.9 mA•cm −2 ) has the largest normalized current, followed by Cu (68.2 mA•cm −2 ) and Cu-18C (64.5 mA•cm −2 ). Therefore, Cu-12C presents a higher C2+ Faradaic efficiency, which is not due to the larger ECSA, but due to the higher mass transport efficiency. As shown in Supplementary Table 5, stronger hydrophobicity of electrode leads to smaller ECSA. However, Cu-12C (93.9 mA•cm −2 ) has the largest normalized current, followed by Cu (68.2 mA•cm −2 ) and Cu-18C (64.5 mA•cm −2 ). Therefore, Cu-12C presents the higher C2+ Faradaic efficiency, which is not due to the larger ECSA, but due to the higher mass transport efficiency.

Specific Comments R3-6: Fig3 c, CO2RR vs CORR is quite surprisingly very different for C2H4.
Response: Thanks to the reviewer for the helpful comment very much. The selectivity of ethanol and ethylene first increases and then decreases (like a volcano) with increasing *CO coverage (Nat. Catal., 2020, 3, 75-82;Nat. Catal., 2019, 2, 1124-1131. As we mention above, the generation of ethanol is in relevance with *CO coverage and *H coverage simultaneously, both of which need to be balanced to achieve high selectivity of ethanol. In our CORR experiment, the direct introduction of CO could lead to excessive *CO coverage, but the *H coverage still be preserved at a certain level as CO2RR experiment. Thanks for the reviewer's kind suggestion, and we reduce the CO feeding concentration in updated CORR experiment, and more ethanol in CORR can be obtained, which can better support our proposed mechanism. The updated CORR result and discussion is shown in page 9 and 10 in revised manuscript. "…Like in the CO2RR, the direct coupling between two *CO species in the CORR is widely accepted as the major C-C coupling mechanism. Moreover, *CO coverage is determined by the local concentration of CO near catalyst. Thus, the coverage of key intermediate *CO can be improved by directly using CO as reactants. When the Cu catalyst has similar contact angle, the promotion of ethanol production is more obviously than that of ethylene during CORR. In comparison, the similar phenomenon can be observed experiments on CuAg catalyst, which can generate more CO during performing CO2RR (Supplementary Fig. 32). Therefore, increasing *CO coverage will benefit to ethanol generation…" Fig. 3 │Effect of controllable wettability on *CO coverage via CO2 mass transport. a, Top row: schematics display the CO2RR configuration with (right column) or without (left column) alkanethiol modification. The red dotted box is the simulation area. Second row: comparison of modeled gas availability along the catalyst surface with (right column) or without (left column) alkanethiol modification. Gas availability dramatically increases via alkanethiol-derived hydrophobic environment. b, Local CO2 concentration versus time during the CO2RR of 100 mA cm -2 of modeled Cu, Cu-12C and Cu-18C interfacial environments, respectively. A stronger the hydrophobicity indicates a faster CO2 mass transport. c, Comparison of ethylene and ethanol Faradaic efficiency enhancement values for CO2RR versus CORR on Cu, which indicates the improvement of *CO coverage is more favorable for the ethanol pathway than for ethylene. In-situ ATR-SEIRAS spectra of d, Cu and (e) Cu-12C, revealing that higher *CO coverage can be maintained on Cu electrodes after hydrophobic treatment. Wherein the stretching band at ~2070 cm -1 corresponds to the stretching band of COL on Cu surface. Response: Thanks to the reviewer for the kind comment. We have revised these mistakes on page 9-10 in the revised manuscript.

Specific Comments
"… Our data (Fig. 2) also imply that the *CO/*H ratio could be controlled by tuning the local CO2/H2O ratio through changing wettability of the catalyst…The models with and without the alkanethiol modification layer were established to quantity dissolved CO2 in solution (Fig. 3a)…" "…To investigate the local CO2 concentration of different interfacial wettability, control samples were prepared with different gas-liquid-solid contact, enabling in situ measurements with fluorescence electrochemical spectroscopy (FES) at 100 mA cm -2 chronopotentiometry (Fig.  3b)…" "…The hydrophobic treatment also can promote CO diffusion in CO reduction reaction. Thus, it can be excluded that the limited CO2 mass transport at the hydrophilic Cu electrode is entirely due to the neutralization of CO2 with the electrolyte (Fig. 3c, Supplementary Fig. 23)…" "…The local CO2 concentration can further influence the coverage of *CO, which affects the reaction pathways toward C2+ products. Thus, in-situ ATR-SEIRAS was employed to further evaluate the impact of interfacial wettability on the adsorption of intermediates (Fig. 3d, e)..." Specific Comments R3-9: "It is known that the mass transport of CO2 (local CO2 concentration) can affect the coverage of *CO2 (a precursor of *CO), *CO, and *H, which affects the reaction pathways toward ethylene and ethanol in further" From where is this known? How is mass transport linked to the coverage of intermediates? Coverage is usually given from the binding energies and the chemical potentials -not mass transport properties.

Response:
We thank the reviewer for insightful comment. We apologize that we did not cite enough papers to support this statement, as well as distinguish thermodynamic and kinetic factors well in the manuscript, which led to misleading. The relevance between local CO2 concentration ([CO]) and the surface coverage of CO (θCO) is shown as equation (Nat. Catal., 2019, 22, 1124-1131Joule, 2020, 4, 1-17): where θ is the coverage of free surface sites, ECO is the CO adsorption energy on the surface, R is the ideal gas constant and T is the temperature. The coverage of intermediate (θCO) is a function of CO adsorption energy (ECO) and local CO concentration ([CO]). A promising means of controlling CO coverage is by the CO partial pressure (pCO) or local CO concentration ([CO]), where the local CO concentration correlates with pCO ([CO]) by Henry's law. In this work, the catalytic reaction is controlled by kinetics (mass transport) without changing thermodynamics. Specifically, we strove to increase the reaction rate of one step without strongly modulating the others, by judiciously increasing the coverage of one key intermediate, yet not interfering with the electronic structure (hence binding strength) of Cu. Therefore, catalytic reaction is not only related to thermodynamics, but also to kinetics.
We have cited related papers to support this statement and added associated analysis on page 9 and page 16 in the revised manuscript.
"It is known that the mass transport of CO2 (local CO2 concentration) can affect the coverage of *CO2 (a precursor of *CO), *CO, and *H, which affects the reaction pathways toward ethylene and ethanol in further 5,20,42  "…Further, the reaction pathways on catalyst surfaces with different wettability can be understood (Fig. 6c). The surface coverage of adsorbed *COx (θCOx, x=1 or 2) on the catalyst is proportional to the local concentration of COx ([COx]), which is given as Equation 1 5,42 : where θ is the coverage of free surface sites, ECOx is the COx adsorption energy on the surface, R is the ideal gas constant and T is the temperature. The coverage of intermediate (θCOx) is a function of CO adsorption energy (ECOx) and local CO concentration ([COx]). To vary the *CO coverage, changing the local concentration of CO at catalysts layer rather than *CO adsorption energy is therefore a promising approach. Compared with the classical modulation of active site, the novelty of our method is that the CO2RR is completely controlled by kinetics (mass transport) through controllable interfacial wettability ( Supplementary Fig. 37, 38)…" Specific Comments R3-10: "To circumvent this issue, computational fluid dynamic (CFD) simulations were employed to investigate the mass transport of CO2 and H2O in the catalyst layers related to *CO and *H" please consider the usage of *CO and *H throughout the manuscript.
Response: Thanks to the reviewer for valuable comment. We are sorry that did not explain how mass transport (local CO2 concentration) is linked to the coverage of intermediates, which confuses readers. Further, we did not distinguish thermodynamic and kinetic factors well in the manuscript. As mentioned above, the coverage of intermediate is a function of CO adsorption energy and local CO concentration. The surface coverage of adsorbed *CO on the catalyst is proportional to the local concentration of CO2 (Nat. Catal., 2019, 22, 1124-1131Joule, 2020, 4, 1-17). Thus, catalytic reaction is not only related to thermodynamics, but also to kinetics. To avoid misleading, we have added detailed explanations of the relevance between local CO2 concentration and the coverage of intermediates on page 16 in the revised manuscript. Further, we have carefully considered and revised the usage of *CO and *H throughout the manuscript. Specifically, we added "kinetic-controlled" in front of *CO or *H to distinguish thermodynamic and kinetic factors, and added "local CO2 concentration" or "mass transport of CO2 and H2O" to link with *CO and *H. For example: "…To circumvent this issue, computational fluid dynamic (CFD) simulations were employed to investigate the mass transport of CO2 and H2O in the catalyst layers related to kinetic-controlled *CO and *H." "…Further, the reaction pathways on catalyst surfaces with different wettability can be understood (Fig. 6c). The surface coverage of adsorbed *COx (θCOx, x=1 or 2) on the catalyst is proportional to the local concentration of COx ([COx]), which is given as Equation 1 5,42 : where θ is the coverage of free surface sites, ECOx is the COx adsorption energy on the surface, R is the ideal gas constant and T is the temperature. The coverage of intermediate (θCOx) is a function of CO adsorption energy (ECOx) and local CO concentration ([COx]). To vary the *CO coverage, changing the local concentration of CO at catalysts layer rather than *CO adsorption energy is therefore a promising approach. Compared with the classical modulation of active site, the novelty of our method is that the CO2RR is completely controlled by kinetics (mass transport) through controllable interfacial wettability ( Supplementary Fig. 37, 38)…" Specific Comments R3-11: "The coverage of key intermediate *CO can be improved by directly using CO as reactants." a statement without any explanation. Why is this?
We have added associated explanations on page 10 and 16 in the revised manuscript to help readers better understand this statement.
"…Like in the CO2RR, the direct coupling between two *CO species in the CORR is widely accepted as the major C-C coupling mechanism. Moreover, *CO coverage is determined by the local concentration of CO near catalyst. Thus, the coverage of key intermediate *CO can be improved by directly using CO as reactants…" "…Further, the reaction pathways on catalyst surfaces with different wettability can be understood (Fig. 6c). The surface coverage of adsorbed *COx (θCOx, x=1 or 2) on the catalyst is proportional to the local concentration of COx ([COx]), which is given as Equation 1 5,42 : where θ is the coverage of free surface sites, ECOx is the COx adsorption energy on the surface, R is the ideal gas constant and T is the temperature. The coverage of intermediate (θCOx) is a function of CO adsorption energy (ECOx) and local CO concentration ([COx])…" Specific Comments R3-12: "As another key intermediate, *H derives from the bulk solution, which is affected by H2O transport." Please consider the usage of *H (there is no *H in solution).
Response: Thanks to the reviewer for helpful comment. We strongly agree with the reviewer that there is no *H in solution. Further, CO2 reduction requires water as a H source for hydrocarbon production (Science, 2020, 367, 661-666;Nat. Mater., 2019, 18, 1222-1227. Thus, the *H intermediate on surface is converted from the bulk solution. We have reconsidered and revised the usage of *H on page 12 in the revised manuscript.
"As another key intermediate, the *H intermediate on surface is converted from the bulk solution, which is affected by interfacial H2O transport…" General Comments R2: Authors demonstrated a great effort towards responding to the questions. Most of the comments are addressed, however, in a few cases the responses are not yet satisfactory, detailed below: Response: We appreciate the reviewer very much for the positive evaluation of our work. We have carefully revised the manuscript in accordance with the reviewer's comments. The reviewer's comments are extremely helpful and have made our work more critical.
Specific Comments R2-1: Although nothing is observed using Raman, the question still remains there, and the reviewer believes that the study lacks a direct evidence showing *H coverage.

Response:
We thank the reviewer for the valuable comment very much. We apologize that we did not explain speculative reasons in the manuscript, which leads to misleading. We have added caveats in conclusions drawn from Raman and discuss the limitation. Raman may be useful for metals with strong binding with *H, such as Pt and Pd. However, the peak of *H on Cu cannot be observed using Raman due to the extremely weak adsorption of *H on Cu. The advanced characterization methods for directly prove *H coverage are expected to better understand the CO2RR process. Thus, a direct evidence for *H coverage cannot be supported by Raman, we provided a speculative explanation based on the experimental results.
An independent means of controlling intermediate coverage (*CO, *H) is by changing the local concentration of reactants (CO2, H2O). For example, the link between CO coverage (θCO) and the local CO concentration is given as following Equation 1(Nat. Catal., 2019, 22, 1124-1131, Joule 2020Nat. Commun., 2021, 12, 36): Where θ is the coverage of free surface sites, ECO is the CO adsorption energy on the surface, R is the ideal gas constant and T is the temperature. The coverage of intermediate (θCO) is a function of CO adsorption energy (ECO) and local CO concentration ([CO]). At equilibrium, the surface coverage of intermediate (*CO, *H) is directly proportional to the concentration of reactants (CO2, H2O). Therefore, it is reasonable to indirectly speculate the *H coverage rate through water content.
In this work, the speculative reasons for *H coverage comes from the two methods mentioned above, namely indirect proof by water content and *CO coverage, and the conclusions (link between wettability and *H coverage) drawn by these two methods are the same. Thanks to the kind reminder from the reviewer, we have realized that we lack an explanation for speculation of *H coverage, and the language when discussing the mechanism is not very reasonable. Therefore, we have added caveats in conclusions drawn from Ramanand discuss the limitation (page 13 and page 22 in revised manuscript; page 36 in Supplementary Information)

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Unfortunately, the peak of *H on Cu cannot be observed by in-situ Raman spectra ( Supplementary Fig. 35) or in-situ ATR-SEIRAS spectra (Fig. 3d, 3e) due to the extremely weak adsorption of *H on Cu. At present, *H coverage can be investigated indirectly through *CO coverage or H2O content 18,46-49 . *H and *CO occupy most of the Cu surface sites, resulting in direct competition between *H and *CO for surface sites 47 . When the surface maintains a high *CO coverage, the corresponding *H coverage will decrease 12,18,46,47 . According to the results of in-situ ATR-SEIRAS spectra (Fig. 3d, 3e) and in-situ Raman spectra ( Supplementary Fig. 35), higher hydrophobicity indicates higher *CO coverage, thus the corresponding *H coverage is lower. Further, the surface coverage of intermediate (*CO, *H) is directly proportional to the concentration of reactants (CO2, H2O). Therefore, the *H coverage can be deduced indirectly through water content.

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In-situ Raman spectroscopy measurements. In-situ Raman spectroscopy was carried out in a custom-designed flow cell, which was manufactured by Gaossunion Co., Ltd., Tianjin. The electrode was encased in a PEEK fitting, with an exposed circular geometric surface area of ~1 cm 2 . A platinum wire and an Ag/AgCl electrode (saturated KCl, Gaossunion Co., Ltd., Tianjin) were used as the counter and the reference electrode, respectively. The counter electrode is separated from the working electrode by an anion exchange membrane (FAA-3-PK-75, Fumatech) to avoid cross-contamination. In situ Raman spectroscopy was performed with a Raman microscopy system (LabRAM HR Evolution, Horiba Jobin Yvon). A 785 nm laser served as the excitation source. Electrochemical measurements were carried out with a potentiostat (CompactStat.e20250, IVIUM).